Ron Guth:
According to official records, the Mint produced 725 Proof Silver Dollars in 1866. Most of the original mintage still survives, usually in PR63 and PR64. Approximately one-fourth of the certified Proofs are Cameos, and less than ten percent are Deep Cameos. The finest example is a single, spectacular PCGS PR69CAM.

According to Breen and Bowers, three die pairs were used to strike these Proofs, which is a surprisingly large number of different die pairs for such a low mintage. In fact, this may not be correct, as one of the die pairs was also used to strike copper versions (Judd-541) for sale to collectors. Both Bowers and Breen claim that this die pair is the most common of the year. However, attributing numerous examples indicates that the opposite is true. It may also be true that no silver examples exist from this die pair, despite Breen's assertions to the contrary. If only copper versions were made, then only two die pairs were used to strike the silver Proofs. Research is ongoing.